The photolithographic process is one of the most important in semiconductor device fabrication. It transfers the designed pattern from a mask or reticle to photoresist that temporarily coats the wafer surface. A stepper is the most commonly used tool to pattern the photoresist coated on the wafer surface, by exposing the photoresist with ultraviolet (UV) light or deep UV light to induce photochemical reactions. It is usually the most expensive tool in advance semiconductor fabrication foundries as well.
A stepper is more specifically an aligner tool that aligns and exposes one die, or a small number of dies, at a time. The tool steps to each subsequent die on the wafer, until the desired pattern covers the entire wafer. Thus, a step and repeat operation causes a desired semiconductor pattern of a reticle to be transferred to a semiconductor wafer placed on a stage or table, where the reticle pattern is repeatedly imaged over the wafer surface until the entire surface is filled.
FIG. 1 shows an example stepper 100. The stepper 100 can be, for instance, one of those manufactured and sold by ASML, of The Netherlands. Of particular interest is that the stepper 100 has two axes, a q-axis 102 and the p-axis 104. Light from each of the q-axis 102 and the p-axis 104 is directed towards a wafer resting on the wafer table 106. Offset plane plates 108 and 110 can be tilted to effectively introduce a tilt of the wafer resting on the wafer table 106, in either or both of the two axes 102 and 104, respectively. That is, tilting the offset plane plates 108 and 110 is effectively the same as tilting the wafer resting on the wafer table 106. The terminology semiconductor wafer tilt as used herein encompasses both such effective wafer tilt as well as actual wafer tilt.
Furthermore, the quad cells 112 and 114 determine which part of the semiconductor pattern being imaged is projected onto which part of the semiconductor wafer resting on the wafer table 106. Thus, rather than physically rotating the wafer resting on the wafer table 106, the quad cells 112 and 114 can instead be appropriately manipulated to cause the same effect as such rotation, which is encompassed under the terminology effective semiconductor wafer rotation as used herein. The terminology semiconductor wafer rotation as used herein encompasses both actual and effective wafer rotation. FIG. 2 shows the path 200 of a white light beam as it is projected through either of the quad cells 112 and 114 of the stepper 100 of FIG. 1, as can be appreciated by those of ordinary skill within the art.
One difficulty with steppers, such as the stepper 100, relates to semiconductor patterns that have asymmetrical topography. The leveling sensor detects the height difference of the asymmetrical topography. The leveling system then compensates for the topographical height difference between the areas of dense pattern density and sparse pattern density by actually or effectively inclining the plane of the stage on which the wafer rests, thereby inducing wafer tilt. For instance, such wafer tilt may be effectively induced by rotating the offset plane plates 108 and 110 for the q-axis 102 and the p-axis 104, respectively, of the stepper 100 of FIG. 1.
Wafer tilt, however, can cause asymmetrical field-related defocus. More specifically, within in the second metal layer of certain semiconductor patterns, such as certain semiconductor memory-related patterns, the photoresist profile may be rounded on one side or the other, causing less than desirable photolithographic imaging, which can affect the ultimate quality of the semiconductor devices being fabricated. Current solutions to this problem center on simulating the semiconductor wafer tilt and correcting the wafer tilt within the stepper itself.
However, attempting to simulate the wafer tilt can be difficult to accomplish. Moreover, correcting the wafer tilt within the stepper can undesirably increase semiconductor fabrication processing times. Both of these can increase the cost of fabricating semiconductor devices, particularly semiconductor memories. This can negatively affect the semiconductor foundry where the fabrication occurs. This is especially the case for semiconductor memories, where profit margins are relatively low.
Therefore, there is a need for correcting semiconductor wafer tilt induced by the leveling sensors of steppers. More specifically, there is a need for correcting such semiconductor wafer tilt that results when imaging photolithographic patterns having asymmetrical topographies onto semiconductor wafers. For these reasons, as well as other reasons, there is a need for the present invention.